An engineering marvel! How the Queensferry Crossing became the UK’s tallest bridge

Lee Bell Read more August 29, 2017

Six years in the making and described as “an incredible feat of civil engineering”, the Queensferry Crossing – a new £1.35 billion, 1.7-mile bridge over the river Forth in Scotland – in now open.

Touted as “one of the world’s great bridges” by the man behind the project, Michael Martin, the new crossing will replace the Forth Road Bridge as the main road route between Edinburgh and Fife, and is expected to carry over 24 million vehicles a year.

The existing Forth Road Bridge that the Queensferry Crossing replaces was built in 1964 and has since been dogged by maintenance problems. However, it will continue to be used by cycles, pedestrians and eventually buses. The iconic 19th Century Forth Bridge, which carries the railway over the river, also still stands, and lies just a mile away.

Building the Queensferry Crossing

Preparation and challenges

Widely considered as the centre of one of the largest Scottish infrastructure projects in a generation, Queensferry Crossing is one of few major developments of its type and scale that has gone from first steps to completion in under 10 years.

After Transport Scotland spent more than three years designing, procuring and managing the project through to approval by the Scottish Parliament, contractors arrived on site in the summer of 2011, with many engineering challenges ahead.

queensferry-crossing-foundations

One of the most challenging elements of the Queensferry Crossing build was said to be the underwater foundations of the three main towers and viaduct piers. Constructing these involved sinking huge steel caissons to the bed of the cold and mighty river Forth, then digging down through the silt and till to bedrock, the project’s website explains. This all had to be achieved in poor visibility and the difficult waters of the Firth of Forth, as the river widens to meet the North Sea.

Further production of the actual bridge also proved very challenging. The project’s marine yard became a production line for fabricating the Queensferry Crossing’s deck sections, where casting the reinforced concrete deck for each steel box section trebled the overall deck section weight from around 250 tonnes to 750 tonnes. These enormous sections then had to make their way out across the water by barge to meet their respective towers. The Queensferry Crossing YouTube channel shows some of the major construction and engineering projects carried out to build the bridge.

“From their marine yard home, these 750-tonne segments were barged out into the Forth to be lifted by special movable hydraulic lifting equipment called erection travellers,” those behind the project stated. “The process was highly sensitive to wind and tide, taking around four hours to carefully lift each section around 80 metres to deck level.

“They were then held in place by the traveller before cable stays could be attached, tensioned and the section welded and ‘stitched-in’ with concrete.”

Each deck fan was then created with sequential north and south lifts that balanced the structure.

According to those behind the project, the Centre Tower of the bridge is the tallest of the three Queensferry Crossing towers, towering at 210 metres high, and making the Queensferry Crossing the tallest bridge in the UK. And even before being connected to the rest of the structure, the Centre Tower was breaking records; its fan was the longest free-standing balanced cantilever ever constructed.

Queensferry Crossing: Finishing touches and opening

The final stage of the Queensferry Crossing construction included completion of many operational and aesthetic final touches to get the new bridge ready for the public opening today. Until recently, the deck of the Queensferry Crossing appeared as bare concrete – but road surfacing work added recently gave it the familiar road materials and markings. Wind-shielding technology was also added to finish the build, designed to keep the bridge open in high winds.

Now finished, the Queensferry Crossing was opened to traffic in both directions in the early hours of this morning. A rolling roadblock was put in place to stop traffic driving across the Forth Road Bridge with drivers redirected across the Queensferry Crossing.

The Northbound carriageway was opened first, with the southbound carriageway opened about 45 minutes later. The bridge will be fully open to traffic for the rest of the day and the following day before closing again to allow 50,000 members of the public a “once in a lifetime” chance to walk over the new bridge on Saturday and Sunday.

Queensferry Crossing by the numbers

Once the Queensferry Crossing is open to the public more than 24 million vehicles are expected to use the crossing each year. Here are some other headline numbers to showcase the magnitude of the project:

£1.45 billion – the cost of the project came in under the original estimate of £4.2bn.
10 million – the number of man hours involved in construction.
35,000 – the number of tonnes of steel used to build the Queensferry Crossing.
7,000 – the number of tonnes of steel fitted in the north and south viaducts.
23,000 – the number of miles of cables used to support the bridge.
122 – the number of sections in the bridge deck.
210 – at 210 metres tall, the Queensferry Crossing is the tallest structure in Scotland.